Mary is not part of the Trinity. The word "impeccable" does not appear in the Catechism, it describes Her as the "exemplary realization" of the Church.
And??? so what??
Does the church teach she is sinless or not?
BTW I do not much care if your catechism has the word in it, the question is , is that an attribute of God/Christ?
S: (n) faultlessness, impeccability (the quality of being without an error or fault)
S: (n) impeccability (the quality of being exempt from sin or incapable of sinning)
I would say that defines God, and lists as one of His Incommunicable” attributes